1. The Field of the Invention
The present invention relates generally to x-ray tubes. More particularly, embodiments of the present invention relate to an x-ray tube cooling system that increases the rate of heat transfer from the x-ray tube to a cooling system medium, thereby significantly reducing heat-induced stress and strain in x-ray tube structures and extending the operating life of the device.
2. The Relevant Technology
X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly used in areas such as diagnostic and therapeutic radiology; semiconductor manufacture and fabrication; and materials analysis and testing.
While used in a number of different applications, the basic operation of x-ray devices is similar. In general, x-rays, or x-ray radiation, are produced when electrons are produced and released, accelerated, and then stopped abruptly. The typical basic x-ray tube has a cathode cylinder with an electron generator, or cathode, at one end. Electrical power applied to a filament portion of the cathode generates electrons by thermionic emission. A target anode is axially spaced apart from the cathode, and is oriented so as to receive electrons emitted by the cathode. Also present is a voltage source that is used to apply a high voltage potential between the cathode and the anode.
In operation, the high voltage potential is applied between the cathode and the anode, which causes the thermionically emitted electrons to accelerate away from the cathode and towards the anode in an electron stream The accelerating electrons then strike the target anode surface (or focal track) at a high velocity. The target surface on the anode is composed of a material having a high atomic number, and a portion of the kinetic energy of the striking electron stream is thereby converted to electromagnetic waves of very high frequency, i.e., x-rays. The resulting x-rays emanate from the target surface, and are then collimated through a window formed in the x-ray device for penetration into an object, such as a patient""s body. As is well known, the x-rays that pass through the object can be detected and analyzed so as to be used in any one of a number of applications, such as x-ray medical diagnostic examination or material analysis procedures.
A percentage of the electrons that strike the anode target surface do not generate x-rays, and instead simply rebound from the surface. These are often referred to as xe2x80x9cback-scatterxe2x80x9d electrons. In some x-ray tubes, some of these rebounding electronsxe2x80x94still traveling at relatively high velocitiesxe2x80x94are blocked and collected by a shield structure that is positioned between the cathode and the anode so the rebounding electrons do not re-strike the target surface of the anode. In this way, the rebounding electrons are prevented from reimpacting the target anode and producing xe2x80x9coff-focusxe2x80x9d x-rays, which can negatively affect the quality of the x-ray image. Some of the rebounding electrons may also impact the interior of the cathode cylinder.
While such a shield structure may prevent rebounding electrons from re-striking the anode target, its use can result in additional problems that can ultimately damage the x-ray tube device, and shorten its operational life. In particular, the high kinetic energy of the rebounding electrons is converted to thermal energy by the impact of those electrons on the shield structure or on the interior of the cathode cylinder. Due to the high level of kinetic energy of the electrons, the thermal energy produced by these impacts is significant and typically results in very high temperatures in the x-ray tube structures. These thigh temperatures, in combination with the high temperatures also being generated at the target anode, cause thermal stresses in the structures (including the cathode cylinder and the shield) and structure joints that can, especially over time, lead to various structural failures in the x-ray tube assembly. Moreover, because the rebounding electrons impact some portions of the cathode cylinder and shield structure with relatively greater frequency than other portions the heat produced by the rebounding electrons is not evenly distributed. Accordingly, the different heat regions are collectively characterized by varying rates of thermal expansion, resulting in mechanical stresses that can also damage the x-ray tube device, especially over numerous operating cycles.
For instance, mechanical stress and strain is induced when the cooler part of the structure resists the expansion of the hotter portion of the structure. The level of stress and strain is relatively insignificant at low temperature differentials. However, non-uniform expansion produced by high temperature differentials induces destructive mechanical stresses and strains that can ultimately cause a mechanical failure in the part. Moreover, these stresses are especially damaging to joints between attached components.
Because such high temperatures can cause destructive thermal stresses and strains in the shield structure, the cathode cylinder, and in other parts of the x-ray device, attempts have been made to minimize thermal stress and strain through the use of various types of cooling systems. However, previously available x-ray tube cooling systems have not been entirely satisfactory in providing effective and efficient coolingxe2x80x94especially in the regions of the shield structure and cathode cylinder.
In order to dissipate the high heat present, x-ray tubes have typically utilized some type of liquid cooling arrangement. In such systems, at least some of the external surfaces of the cathode cylinder are placed in direct contact with a circulating coolant, which facilitates a convective cooling process. Often however, this approach is not satisfactory for cooling an adjacent shield structure, which has a limited external surface area, and, because it is exposed to extremely high temperatures from rebounding electrons, is unable to efficiently transfer significant amounts of heat by convection to the coolant.
To address this problem, shield structures have been fashioned with internal cooling passages through which a coolant stream is circulated. Thus, the shield structure gives up heat primarily by convection to the coolant which flows through its interior. This approach has not been entirely satisfactory either. Due to the limited size of such cooling passages, only a limited amount of heat can be absorbed by the coolant, and consequently the shield structure may not be adequately cooled. Thus, x-ray devices of this sort may experience greater failure rates and shorter operating lives due to repeated exposure to higher temperatures and resultant stresses.
Also, in systems of this sort, the coolant must be capable of absorbing significant amounts of heat in order to preclude harmful thermal stresses and strain in the shield structure and cathode cylinder. However, with current designs, the circulated coolant eventually, and often prematurely, experiences thermal breakdown and is no longer able to effectively remove heat from the x-ray tube. Again, this translates into an x-ray device that is more subject to failure and that typically has an overall shorter operating life.
Currently available cooling system designs are lacking in another respect as well. As noted, heat produced within the x-ray tube is not evenly distributed. However, currently available cooling systems are not capable of removing heat from certain higher-temperature areas of the x-ray tube faster than cooler areas. Instead, the rate of heat transfer is fairly constant throughout the x-ray tube in existing systems. As such, those regions that are exposed to higher temperatures are not adequately cooled, and experience a greater failure rate.
There are additional problems in existing x-ray tube designs caused by excessive operating temperatures. In particular, the high operating temperatures are especially destructive to the connection points between the various component parts of the x-ray tube device. For instance, the cathode cylinder is fashioned as a single integral part that must be attached to the shield structure. The shield structure is then affixed to the housing, or xe2x80x9ccan,xe2x80x9d that encloses the x-ray tube assembly. Typically, these attachments are accomplished by way of a weld or braze joint. However, in prior art systems, these joints have been implemented in a manner that is especially vulnerable to the thermal and mechanical stresses present, and often fail prematurely. Thus, efficient removal of heat, as well as robust joint attachments between component parts is critical to maintaining structural integrity and increased operating life of the x-ray device.
Thus, there is a need in the art for a cooling system that can be used to efficiently and effectively remove heat from the x-ray tube, and especially in the areas of the cathode cylinder and the adjacent shield structure. Moreover, it would be desirable to have a system that provides sufficient heat removal to reduce the level of thermal and mechanical stresses otherwise present within the cathode cylinder and shield, and that would thereby increase the overall operating life of the x-ray tube and x-ray device. Likewise, the system should prevent heat-related damage from occurring in the materials used to fabricate the cathode cylinder and shield assembly, and should reduce structural damage from occurring between joints and/or attachment points between the various structural components. Joints between components should be more robust, and able to withstand high temperatures. Also, it would be desirable if the system could effectively remove heat at a higher rate from those areas of the system that experience higher temperatures than other portions, and thereby reduce the occurrence of varying thermal regions.
It is therefore a general objective of the present invention to provide an improved x-ray tube cooling system that addresses the aforementioned problems in the prior art systems.
More particularly, it is a primary object of the present invention to provide an improved x-ray tube cooling system that enhances the convective and conductive heat transfer from components of the x-ray tube to a cooling system coolant, and that is especially efficient in removing heat generated as a result of back scattered electrons within the x-ray tube.
A related objective of the present invention is to provide a cooling system that reduces temperature levels present within x-ray tube components and the coolant, thereby reducing the incidence of failure within the x-ray tube due to thermal stresses and increasing the overall operating life of the x-ray tube.
Another objective of the present invention to provide an improved x-ray tube cooling system in which coolant is circulated through passages formed within a shield structure so as to more efficiently remove heat by convection from the shield structure.
Yet another object of the present invention to provide an improved x-ray tube cooling system which utilizes a shield structure that has increased internal and external surface areas in contact with the cooling system coolant, thereby improving the efficiency and rate at which heat is removed from the shield structure.
Still another objective of the present invention is to provide a cooling system in which areas of the shield structure that have a higher thermal content are cooled at a rate higher than those portions of the shield structure having a lower thermal content.
Another objective of the present invention is to provide improved brazed joints between structures of the x-ray tube that are better able to withstand the thermal and mechanical stresses present within an operating x-ray tube.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.
Briefly summarized, the foregoing objects and advantages are provided with an improved x-ray tube cooling system A preferred embodiment of the system includes a reservoir containing a liquid coolant that is continuously circulated by way of a heat exchanger device. Disposed within the coolant reservoir is an x-ray tube, which consists of a cathode cylinder having an electron source, such as a cathode head assembly, disposed therein. The x-ray tube is also comprised of an evacuated housing that encloses an anode having a target surface capable of receiving electrons emitted by the electron source. Disposed between the cathode cylinder and the x-ray tube housing is a shield structure. The shield structure defines an aperture through which electrons are passed from the electron source to the target surface to generate x-rays. Moreover, the shield structure provides an electron collection surface, that prevents electrons that rebound from the target surface from re-striking the target.
In a preferred embodiment, at least one fluid passageway is formed within the shield structure. The fluid passageway receives coolant from the reservoir from an inlet port, which then passes through the passageway so as to absorb heat generated in the shield structure, including heat generated as a result of rebounding electrons striking inner surfaces of the shield.
Preferred embodiments of the cooling system also include a plurality of extended surfaces, or cooling fins, that are affixed to the outer surface of the shield structure. Coolant exiting the fluid passageway is allowed to flow across the extended surfaces, which are oriented in a manner so as to conduct heat from the shield to the coolant.
In one preferred embodiment, the cooling system also includes means for augmenting the heat transfer capability of the fluid passageway. In an illustrated embodiment, this means is comprised of a plurality of microgrooves formed inside the fluid passageway cooperatively defined by the shield structure and the aperture disk. The microgrooves serve to increase the surface area of the fluid passageway through which the coolant flows and thereby effect a relative increase in the rate of heat transfer from the shield structure to the coolant. Additionally, the microgrooves also improve the efficiency of multiphase heat transfer, beyond the improvement attributable simply to the increase in surface area, by enhancing the mechanism by which ebullition heat transfer, i.e., nucleate boiling occurs.
In an alternative embodiment, the aforementioned means for augmenting the heat transfer capability of the fluid passageway comprises a coiled spring that is disposed within the fluid passageway. The spring provides an extended surface that increases the efficiency and rate at which heat is removed from the shield structure by the coolant.
In yet another preferred embodiment, the fluid passageways that are formed within the shield structure are oriented in a manner that permits coolant to flow through a first and a second section of the shield structure. Moreover, the passageways are further oriented such that the heat is transferred away from the first section at a greater rate than in the second section. In this way, those sections (i.e., the first section) having a higher thermal content are cooled at a faster rate than those sections (i.e., the second section) having a lower thermal content. This ensures a more efficient and evenly distributed dissipation of heat, and also helps ensure that the coolant is not overly thermally stressed.
Embodiments of the invention also are disclosed that provide a more structurally sound x-ray tube assembly, and one that is thus better able to withstand the thermal and mechanical stresses present in an operating tube. For instance, an improved braze joint is provided between the shield structure and the x-ray tube housing. In particular, a braze material is placed along a joint formed along both a horizontal and a vertical surface of the shield structure and the x-ray tube housing. This ensures a connection joint that is more structurally sound, and that is able to survive the varying temperatures, and resultant stresses imposed during operation of the x-ray tube.